Method for operating the catalyst of an internal combustion engine

ABSTRACT

An internal combustion engine ( 1 ), especially for a motor vehicle, is described, which has a catalytic converter ( 12 ) which can be loaded with and unloaded of nitrogen oxides. An NOx sensor ( 14 ) is provided with which NOx emissions can be measured downstream of the catalytic converter ( 12 ). The catalytic converter ( 12 ) can be only partially loaded by a control apparatus ( 18 ). An increased NOx emission can be generated forward of or in the catalytic converter ( 12 ) by the control apparatus ( 18 ). A conclusion can be drawn as to the operability of the catalytic converter ( 12 ) by the control apparatus ( 18 ) from NOx emissions measured downstream.

FIELD OF THE INVENTION

The invention relates to a method for operating a catalytic converter ofan internal combustion engine, especially of a motor vehicle, whereinthe catalytic converter is loaded with nitrous oxides and unloaded andwherein the NOx emissions downstream of the catalytic converter aremeasured. Likewise, the invention relates to a control apparatus for aninternal combustion engine, especially of a motor vehicle, as well as aninternal combustion engine especially for a motor vehicle.

BACKGROUND OF THE INVENTION

A method of this kind, a control apparatus of this kind and an internalcombustion engine of this kind are, for example, known for a so-calledgasoline direct injection. There, the fuel is injected into thecombustion chamber of the internal combustion engine in a homogeneousoperation during the induction phase or in a stratified operation duringthe compression phase. The homogeneous operation is preferably providedfor the full-load operation of the engine; whereas, the stratifiedoperation is suitable for idle operation and part-load operation. In adirect-injecting internal combustion engine of the above kind, aswitchover takes place between the above-mentioned operating modes, forexample, in dependence upon the requested torque.

Especially for carrying out the stratified operation, it is requiredthat a storage catalytic converter be present with which developingnitrogen oxides can be intermediately stored in order to reduce the sameduring a subsequent homogeneous operation. This storage catalyticconverter is loaded in the stratified operation with nitrogen oxides andis again unloaded in the homogeneous operation. An NOx sensor is mounteddownstream of the catalytic converter for controlling and monitoring.The NOx sensor is provided for measuring NOx emissions of thedischarging exhaust gas.

Parts of the catalytic converter can become defective because of thedeterioration thereof or for other reasons.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for operating acatalytic converter of an internal combustion engine with which a defectof the catalytic converter can be detected.

This object is solved in accordance with the invention with a method ofthe kind mentioned initially herein in that the catalytic converter isonly partially loaded, that an increased NOx emission is generated inadvance or in the catalytic converter and that a conclusion is drawn asto the operability of the catalytic converter from the NOx emissionsmeasured downstream. The object is correspondingly solved with a controlapparatus and an internal combustion engine of the respective typesmentioned initially herein.

The catalytic converter can reduce the increased NOx emissions generatedahead of or therein via the only partial loading of the catalyticconverter. In this way, a lower NOx emission is present at the output ofthe catalytic converter than the initially generated NOx emission. Fromthis reduction, a conclusion is drawn in accordance with the inventionas to the operability of the catalytic converter. If a significantreduction of the initially increased NOx emission is present, then aconclusion can be drawn at least insofar as to a proper operation of thecatalytic converter.

The catalytic converter is, however, recognized as being defective whenan increased NOx emission is measured. In this case, the initiallyincreased NOx emission was not reduced by the catalytic converter. Thisdefines an operation of the catalytic converter which is not proper.

In an advantageous embodiment of the invention, the increased NOxemission is generated ahead of or in the catalytic converter by adischarge thereof. An increased NOx emission arises from the switchoverto the discharge. Since the catalytic converter is not fully loaded,this increased NOx emission arises at the beginning or at least in theinterior of the catalytic converter. In this way, for an intactcatalytic converter, the increased NOx emission can be reduced againwithin the catalytic converter itself. There is then no NOx emission oronly a very slight NOx emission measured by the NOx sensor at the outputof the catalytic converter from which a conclusion can be drawn as to aproper operation of the catalytic converter.

Likewise, it is advantageous, when the increased NOx emission isgenerated ahead of or in the catalytic converter by: a reduction of theexhaust-gas recirculation rate and/or by an early shift of the ignitiontime point and/or by a reduction of the fuel pressure. Alternatively oradditively, the increased NOx emission can be generated by chemical NOxsources wherein, for example, urea or nitrate is introduced ahead of thecatalytic converter. Furthermore, the increased NOx emission can begenerated by a spark plug mounted forward of the catalytic converter.

Of special significance is the realization of the method of theinvention in the form of a control element which is provided for acontrol apparatus of an internal combustion engine, especially of amotor vehicle. A program is stored on the control element which can berun on a computing apparatus, especially on a microprocessor, and issuitable for executing the method in accordance with the invention. Inthis case, the invention is therefore realized by a program stored onthe control element so that this control element, which is provided withthe program, defines the invention in the same way as the method forwhose execution the program is suitable. As a control element, anelectric storage medium is especially applicable, for example, aread-only-memory or a flash memory.

Further features, possibilities of application and advantages of theinvention will become apparent from the description of the embodimentsof the invention which follow and which are shown in the figures of thedrawing. All described or illustrated features form the subject matterof the invention individually or in any combination independently of thecomposition thereof in the patent claims or their dependency as well asindependently of their formulation in the description or theirillustration in the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic illustration of an embodiment of an internalcombustion engine according to the invention;

FIG. 2 shows a schematic diagram of the NOx emissions at the output ofthe catalytic converter of the internal combustion engine of FIG. 1;and,

FIGS. 3a and 3 b show schematic diagrams of the fill level and of theNOx emissions of the catalytic converter of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

In FIG. 1, an internal combustion engine 1 of a motor vehicle isillustrated wherein a piston 2 is movable back and forth in a cylinder3. The cylinder 3 is provided with a combustion chamber 4 which, interalia, is delimited by the piston 2, an inlet valve 5 and an outlet valve6. An intake manifold 7 is coupled to the inlet valve 5 and anexhaust-gas pipe 8 is coupled to the outlet valve 6.

An injection valve 9 and a spark plug 10 project into the combustionchamber in the region of the inlet valve 5 and of the outlet valve 6.Fuel can be injected into the combustion chamber 4 via the injectionvalve 9. The fuel in the combustion chamber 4 can be ignited by thespark plug 10.

A rotatable throttle flap 11 is accommodated in the intake manifold 7and air can be supplied to the intake manifold 7 via the throttle flap11. The quantity of the supplied air is dependent upon the angularposition of the throttle flap 11. A catalytic converter 12 isaccommodated in the exhaust-gas pipe 8 and functions to purify theexhaust gas arising because of the combustion of the fuel.

The catalytic converter 12 is a storage catalytic converter which iscombined with a three-way catalytic converter. The catalytic converter12 is therefore, inter alia, provided for intermediately storingnitrogen oxides (NOx).

The catalytic converter 12 in correspondence to FIG. 1 is assembled fromtwo carriers 13 for the catalytic coating, so-called bricks. An NOxsensor 14 is provided in the exhaust-gas pipe arranged directlydownstream of the catalytic converter 12. The NOx emissions in theexhaust gas flowing out of the catalytic converter 12 are measured viathe NOx sensor 14.

Input signals 19 are applied to a control apparatus 18 and representoperating variables of the engine 1 measured by means of sensors. Thecontrol apparatus 18 generates output signals 20 with which theperformance of the engine 1 can be influenced via actuators orpositioning devices. The control apparatus 18 is, inter alia, providedfor controlling (open loop and/or closed loop) the operating variablesof the engine 1. For this purpose, the control apparatus 8 is providedwith a microprocessor which has a program stored in a storage medium,especially in a flash memory, which is suitable for carrying out theabove-mentioned control (open loop and/or closed loop).

In a first operating mode, a so-called homogeneous mode of the engine 1,the throttle flap 11 is partially opened or closed in dependence uponthe desired torque. The fuel is injected into the combustion chamber 4by the injection valve 9 during an induction phase caused by the piston2. The injected fuel is swirled by the air inducted simultaneously viathe throttle flap 11 and the fuel is thereby distributed essentiallyuniformly in the combustion chamber 4. Thereafter, the air/fuel mixtureis compressed during the compression phase in order to then be ignitedby the spark plug 10. The piston 2 is driven by the expansion of theignited fuel. The arising torque is, in homogeneous operation,dependent, inter alia, on the position of the throttle flap 11. Theair/fuel mixture is set as close as possible to lambda=1 for a lowdevelopment of toxic substances.

In a second mode of operation, a so-called stratified operation of theengine 1, the throttle flap 11 is opened wide. The fuel is injected intothe combustion chamber 4 by the injection valve 9 during a compressionphase caused by the piston 2. The injection of fuel is local into theimmediate vicinity of the spark plug 10 as well as timely at a suitableinterval in advance of the ignition time point. The fuel is then ignitedwith the aid of the spark plug 10 so that the piston 2 is driven in thefollowing work phase by the expansion of the ignited fuel. Thedeveloping torque is dependent substantially on the injected fuel massin stratified operation. The stratified operation is essentiallyprovided for the idle operation and the part-load operation of theengine 1.

The storage catalytic converter of the catalytic converter 12 is loadedwith nitrogen oxides during the stratified operation. In thenext-following homogeneous operation, the storage catalytic converter isagain unloaded and the nitrogen oxides are reduced by the three-waycatalytic converter. The storage catalytic converter takes on sulfurduring its continuous loading and unloading of nitrogen oxides as afunction of time. This leads to a limiting of the storage capability ofthe catalytic converter which is hereinafter referred to asdeterioration.

In FIG. 2, the NOx emissions are plotted as function of time as they aremeasured, for example, by the NOx sensor 14 at the output of thecatalytic converter 12. The NOx emissions likewise increase withincreasing loading of the storage catalytic converter. The operatingmode of the engine 1 is switched over when a pregiven threshold value isreached for which the storage catalytic converter is fully loaded andthe storage catalytic converter is, as already mentioned, again unloadedin homogeneous operation.

An increased NOx emission occurs directly after the switchover of theengine 1 into the homogeneous operation. This is identified in FIG. 2 bythe reference numeral 15. This increased NOx emission 15 arises because,on the one hand, the storage catalytic converter releases the storednitrogen oxides because of the homogeneous operation and that, on theother hand, the three-way catalytic converter is not yet in the positionto convert the released nitrogen oxides to nitrogen and oxygen.Accordingly, the nitrogen oxides flow unchanged out of the catalyticconverter 12 and generate the mentioned increased NOx emission 15.

In FIG. 3a, a curve 16 of the fill level state of the catalyticconverter 12 is plotted as a function of the distance which the exhaustgas travels in the catalytic converter 12 through the two carriers 13.From the curve 16 of the fill level state it can be seen thatessentially only the first one of the two carriers 13 of the catalyticconverter 12 in the exhaust-gas direction is partially loaded withnitrogen oxides while the second carrier 13 is unloaded.

When the catalytic converter 12 has a state which corresponds to thecurve 16 of the fill level state of FIG. 3a, a transition into thehomogeneous operation is carried out by the control apparatus 18.Accordingly, an unloading of the storage catalytic converter isinitiated by the control apparatus 18 even though the storage catalyticconverter is not yet fully loaded in accordance with curve 16.

For a fully loaded storage catalytic converter, the switchover wouldhave the consequence that an increased NOx emission would be measured atthe output of the catalytic converter 12 as shown in FIG. 2. Since,however, the storage catalytic converter is not fully loaded incorrespondence to FIG. 3a, this increased NOx emission does not developat the output of the catalytic converter 12 but essentially only at thestart or in the interior of the catalytic converter 12. This resultsfrom the situation that the catalytic converter is greatly loaded onlyat the start in accordance with FIG. 3a.

In FIG. 3b, the increased NOx emissions are, on the one hand, plotted asa function of the distance which the exhaust gas travels in thecatalytic converter 12 as well as, on the other hand, as a function ofthe time which the increased NOx emissions need in order to travel thedistance through the catalytic converter 12.

As can be seen in FIG. 3b, and as already mentioned, the increased NOxemission is relatively large at the beginning of the catalytic converter12. This large increased NOx emission passes through the catalyticconverter 12 in the direction toward its outlet. Along this path, theincreased NOx emission is converted at least partially into nitrogen andoxygen by the three-way catalytic converter. This leads to the situationthat, as a function of time and along the exhaust-gas path, theincreased NOx emission, which was relatively large at the beginning, isslowly reduced. This is shown in FIG. 3b based on the sequentiallyincreased NOx emissions 17 which become ever smaller.

At the output of the catalytic converter 12 which is shown in FIGS. 3aand 3 b by a broken line, a relatively small increased NOx emissionleaves the catalytic converter 12. This relatively small increased NOxemission goes almost toward zero in a corresponding selection of thecurve 16 of the fill level state.

When both carriers 13 of the catalytic converter 12 are in proper order,then, as described, no or only virtually no increased NOx emission ismeasured anymore by the NOx sensor 14 at the output of the catalyticconverter 12. From this, the control apparatus 18 can conclude as to theproper operation of both carriers 13 of the catalytic converter 12.

If, however, the first carrier 13 of the catalytic converter 12, viewedin exhaust-gas direction, is defective, but the second carrier 13 is inproper order, then the increased NOx emission develops only at the startof the second carrier 13 and can no longer be fully reduced toward zerowithin the second carrier 13. In this case, an increased NOx emission ismeasured by the NOx sensor 14 at the output of the catalytic converter12 from which the control apparatus 18 can conclude the malfunction ofat least one of the two carriers 13 of the catalytic converter 12.

The corresponding situation is valid when the first carrier 13 is inproper order but the second carrier 13 is defective. In this case, theincreased NOx emission does occur at the beginning of the first carrier13, but cannot be reduced completely toward zero without the secondcarrier 13. Therefore, in this case, an increased NOx emission ismeasured by the NOx sensor 14 at the output of the catalytic converter12 and, from this fact, the control apparatus 18 can draw a conclusionas to the defect of at least one of the two carriers 13 of the catalyticconverter 12.

If both carriers 13 of the catalytic converter 12 are defective, thennothing can be measured by the NOx sensor 14 because no nitrogen oxidewas stored in the storage catalytic converter and therefore no increasedNOx emission can develop. This case cannot be distinguished from thefirst explained case wherein both carriers 13 of the catalytic converter12 are in proper order. For this to be distinguishable, other methodsare to be applied which are not described here.

If, proceeding from a fill level state in accordance with FIG. 3a andafter a switchover into the homogeneous operation, an increased NOxemission is measured by the NOx sensor 14, then a conclusion can bedrawn by the control apparatus 18 from this as to a defect of at leastone carrier 13 of the catalytic converter 12.

For detecting the increased NOx-emission, the following can be appliedby the control apparatus 18: the maximum value of the NOx emissionmeasured by the NOx sensor and/or the surface under these NOx emissionsand/or available time constants of these NOx emissions or the like.

It is understood that the above-described method can also be applied toa catalytic converter which has only a single carrier 13 for thecatalytic coating. In this case, because of an increased NOx emission, aconclusion can be drawn as to a defect of at least a part of thiscatalytic converter 12.

In the described method, the increased NOx emission 15 of FIG. 2 isgenerated by a switchover into the homogeneous operation. It is likewisepossible to generate an increased NOx emission also in other ways.Accordingly, an increased NOx emission can be generated by reducing theexhaust-gas recirculation rate and/or by advancing the ignition timepoint or by reducing the fuel pressure. Likewise, an increased NOxemission can be generated by chemical NOx sources where, for example,urea or nitrate can be introduced ahead of the catalytic converter 12.Likewise, an increased NOx emission can be generated in that a sparkplug, which is mounted ahead of the catalytic converter 12, iscorrespondingly driven.

What is claimed is:
 1. A method for operating a catalytic converter ofan internal combustion engine including an internal combustion engine ofa motor vehicle, the method comprising the steps of: generating NOxemissions by operating the internal combustion engine in a lean mode andonly partially loading the catalytic converter with NOx emissions;generating an NOx emission ahead of or in the catalytic converterwhereby the generated NOx emission is increased compared to the NOxemissions generated in the lean mode; measuring NOx emissions downstreamof the catalytic converter; and, drawing a conclusion as the operabilityof the catalytic converter from the NOx emissions measured downstream ofthe catalytic converter.
 2. The method of claim 1, wherein the catalyticconverter is detected as being defective when an increased NOx emissionis measured.
 3. The method of claim 1, wherein the increased NOxemission is generated forward of or in the catalytic converter by anunloading thereof.
 4. The method of claim 1, wherein the increased NOxemission forward of or in the catalytic converter is generated by atleast one of a reduction of the exhaust-gas recirculation rate, anadvancement of the ignition time point and a reduction of the fuelpressure.
 5. The method of claims 1, wherein the increased NOx emissionis generated by chemical NOx sources.
 6. The method of claim 1, whereinthe increased NOx emission is generated by a spark plug mounted forwardof the catalytic converter.
 7. A method for operating a catalyticconverter of an internal combustion engine including an internalcombustion engine of a motor vehicle, the method comprising the stepsof: only partially loading the catalytic converter with NOx emissions;generating an increased NOx emission ahead of or in the catalyticconverter; measuring NOx emissions downstream of the catalyticconverter; drawing a conclusion as the operability of the catalyticconverter from the NOx emissions measured downstream of the catalyticconverter; wherein the increased NOx emission is generated by chemicalNOx sources; and, wherein said chemical sources include urea or nitrateintroduced ahead of the catalytic converter.
 8. A control elementincluding a flash memory for a control apparatus of an internalcombustion engine including an internal combustion engine of a motorvehicle, the control element comprising a program stored thereon whichcan be run on a computing apparatus including on a microprocessor, andthe program is suitable for carrying out a method for operating acatalytic converter of an internal combustion engine including aninternal combustion engine of a motor vehicle, the method comprising thesteps of: generating NOx emissions by operating the internal combustionengine in a lean mode and only partially loading the catalytic converterwith NOx emissions; generating an NOx emission ahead of or in thecatalytic converter whereby the generated NOx emission is increasedcompared to the NOx emissions generated in the lean mode; measuring NOxemissions downstream of the catalytic converter; and, drawing aconclusion as the operability of the catalytic converter from the NOxemissions measured downstream of the catalytic converter.
 9. A controlapparatus for an internal combustion engine including an internalcombustion engine of a motor vehicle, wherein the engine has a catalyticconverter which can be loaded with and unloaded of nitrogen oxides andwherein the engine has an NOx sensor with which NOx emissions can bemeasured downstream of the catalytic converter, the control apparatuscomprising: means for generating NOx emissions by operating the internalcombustion engine in a lean mode and only partially loading thecatalytic converter with NOx emissions; means for generating an NOxemission ahead of or in the catalytic converter whereby the generatedNOx emission is increased compared to the NOx emissions generated in thelean mode; means for receiving and utilizing signals from said NOxsensor which represent NOx emissions measured downstream of thecatalytic converter; and, means for drawing a conclusion as to theoperability of the catalytic converter from the NOx emissions measureddownstream.
 10. An internal combustion engine including an internalcombustion engine for a motor vehicle, the internal combustion enginecomprising: a catalytic converter which can be loaded with and unloadedof nitrogen oxides; an NOx sensor for measuring NOx emissions downstreamof the catalytic converter; and, a control apparatus including: meansfor generating NOx emissions by operating the internal combustion enginein a lean mode and only partially loading the catalytic converter withNOx emissions; means for generating an NOx emission ahead of or in thecatalytic converter whereby the generated NOx emission is increasedcompared to the NOx emissions generated in the lean mode; and, means fordrawing a conclusion as to the operability of the catalytic converterfrom the NOx emissions measured downstream.